My question is, can I use one of the separate parcels as collateral for a construction loan to build a multifamily on it (of course adhering to zoning regulations)?

If you are putting in 50% of the purchase money, guaranteeing the rehab loan alone, and doing all the work, it does not seem fair to you that your investor takes 50% of the profit.Though there are lenders that specialize in second-position rehab loans, most who make these will expect to be in first position.

They may be able to help with a business loan 

Mine are 5% of loan balance if prepaid within 1 year, 3% within 2 years, 1% within 3 years. 

Lastly, is this a renovation loan?

I dont list property I loan money to those that buy these properties and so I see the closing hud and what the brokers charge.. 

I'm sure you can find sample letters/rules online.3.

Heloc I think we could get $175k, maybe DSCR, I am a veteran with service connection disability rating 40% but with my mortgage and car loan not sure I could make debt to income work.  

The current loan is 3.5% so I won’t refinance into a new mortgage.  

You can pay for your buys with normal loans, FHA, or VA loans.